Modern commercial aircraft are designed for maximizing the dollar value of the payload, which can involve maximizing the passenger-carrying capacity. Nevertheless, tradeoffs must be made between such items as seat sizes and amenities to achieve maximized revenue. For example, a passenger cabin, or part of it, having roomier seats and more amenities may command a fare premium sufficient to justify the lesser number of passengers carried.
For many passenger aircraft of circular cross section, the main cabin floor is disposed somewhat below the geometric center to provide a large passenger compartment having the usual seating, overhead storage compartments, and other amenities. Ducts, cables, safety equipment, etc., can run through the small space at the top above the ceiling and overhead bins. The space below the cabin floor, substantially smaller than the cabin area itself, is designed for standardized cargo/baggage containers, as well as equipment bays, structural members, and auxiliary equipment. All in all, the goal is to provide a single, large, attractive passenger cabin with few encumbrances not directly associated with passenger comfort, such as galleys and lavatories.
Ultra-range subsonic airliners may have non-stop flights of a duration of 18 hours or more. On such long flights, it is necessary to provide room for pilot and attendant rest areas. In addition, on such long flights, cargo-carrying weight capability may be limited, such that the lower lobe (also known as the lower deck, lower hold, cargo deck, or cargo pit) remains at least partly unused. Thus, an aircraft configured for maximum commercial efficiency on a short route will not be configured for maximum commercial efficiency on a longer route, and vice versa.